In polarographic sensors and sensing systems of this kind, the diffusion-limited current is measured at a constant voltage applied across the electrodes of the sensor's measuring cell. In an exhaust gas emitted by a combustion process using a lean mixture, this current is dependent upon the oxygen concentration, as long as the diffusion of the gas to the cathode determines the speed of the reaction which is taking place. It has already been proposed that polarographic sensors of this kind be designed in such a manner that both the anode and the cathode are exposed to the gas that is to be measured. The cathode then bears a diffusion barrier, in order to attain functioning in the diffusion-limited current range. The anode is exposed to the gas be measured, without having any such diffusion barrier, so that a constant oxygen partial pressure does not prevail at this anode, which serves as a reference electrode. This is disadvantageous for measurement in the lean range, because at a fuel/air mixture ratio (lambda)=1, the characteristic curve has a current jump, rather than extending over a steady course from lean mixture to rich mixture.